This invention relates to a method of impregnating a fibrous textile material with a plastic resin so as to form an impregnated fibrous textile material which can be hot pressed to produce a laminated composite article.
Fibrous textile materials impregnated with a plastic resin which can be hot pressed to produce laminated composite articles are commonly called "prepregs". Such materials are usually prepared in easy-to-handle sheet or other form by impregnating continuous lengths of fabric or fibers with a suitable moulding resin. The resins employed are usually thermosetting resins which are advanced to the B-stage, or partially polymerized, after impregnation. In this state, the impregnated material can be stored for reasonable lengths of time under normal storage conditions. At the appropriate time, it can be laminated and hot pressed into a desired form under conditions which effect cure of the resin.
Many of the resins employed to impregnate fibrous textile materials to form prepregs are in liquid form and impregnation can be readily effected with such resins by simply immersing the fibrous materials in these resins. Other resins are readily soluble in organic solvents and present no impregnation problems, although solutions of such resins often have a limited shelf life and may present health and environmental problems due to the nature of the solvents employed. A great many resins, however, particularly the thermoplastics, are solids and can only be solubilized with great difficulty, or sometimes not at all.
In order to effect impregnation with these insoluble or difficulty-soluble solid resins, it is usually necessary to employ hot melt techniques. This is usually done by distributing such resin over a fibrous material as a fine powder and then fusing it to infiltrate it around the fibers thereof. However, because of the high surface area of the fibrous material, and because the viscosity of the polymer melt is usually very high, even at elevated temperatures, it is difficult to achieve good wetting of the fibers by such technique, i.e., it is difficult to produce a uniformly impregnated material by such procedure.